Several attempts have been made to provide supercritical-state fuel into the combustion chambers of internal combustion engines to obtain greater fuel efficiency through reduced ignition delay and more complete combustion, while using the improved EGR tolerance to reduce NOx emissions.
Supercritical-state fluid occurs when temperature and pressure reach a point where the fluid is neither a pure gas nor a pure liquid. Above the supercritical point the supercritical-state fluid can have properties that look more like a gas than a liquid, or can have properties that look more like a liquid than a gas, depending on the compound and the temperature and pressure surrounding the compound.
High pressure (over the critical point) creates high density. In an internal combustion engine, high density fuel allows for the creating of sprays with high kinetic energy to form a plume that promotes entrainment and mixing with air and a more complete and fast combustion with good air utilization.
Phase diagrams for CO.sub.2 are shown in FIGS. 1 and 2. In the pressure-temperature phase diagram of FIG. 1, the boiling boundary line 500 separates the gas and liquid regions and ends at the critical point 502, where the liquid and gas phases disappear to become a single supercritical phase. The triple point 506 is a temperature and pressure condition at which all three phases coexist. The density-pressure phase diagram for CO.sub.2, in FIG. 2 allows additional observations. At well below the critical temperature, e.g. 280 K, as the pressure increases, the gas compresses and eventually (at just over 40 bar) condenses into a much denser liquid, resulting in the discontinuity in the line 512 (vertical dashed line) under the liquid-vapor dome. The result is two phases in equilibrium: a dense liquid (with the density indicated at the upper end of the dashed line) 514 and a low density gas (with the density indicated at the lower end of the dashed line) 516. As the critical temperature is approached (curve 518 is the isotherm at 300 K), the density of the gas at equilibrium becomes denser, and the density of the liquid becomes lower. At the critical point 520, (304.1 K and 7.38 MPa (73.8 bar)). There is no difference in density, and the 2 phases become one fluid phase. Thus, above the critical temperature, e.g., 310 K shown as line 522, a gas cannot be liquefied by pressure. At slightly above the critical temperature (310K), in the vicinity of the critical pressure, the density line is almost vertical. A small increase in pressure causes a large increase in the density of the supercritical phase. Many other physical properties also show large gradients with pressure near the critical point, e.g. viscosity, the relative permittivity and the solvent strength, which are all closely related to the density. At higher temperatures, the fluid starts to behave like a gas, as can be seen in FIG. 2. For carbon dioxide at 400 K, the density increases almost linearly with pressure, line 524.
In Table 1 below, it can be seen that the range of density, viscosity and diffusivity for various fluids in their gas and liquid phases have different ranges of properties when the fluids reach their supercritical states.
TABLE 1DensityViscosityDiffusivity(kg/m3)(cP)(mm2/s)Gases  10.01 1-10Supercritical fluids100-10000.05-0.10.01-0.1Liquids1000 0.5-1.00.001
Additionally, there is no surface tension in a supercritical-state fluid, since there is no liquid/gas boundary. A change in pressure and temperature of the fluid can allow one to “tune” the fluid to be more liquid or more gas like. Solubility tends to increase with density of the fluid when held at a constant temperature potentially making solubility another important property of supercritical state fluids. Solubility of material in fluid is another important property of supercritical-state fluids, since solubility tends to increase with density of the fluid when held at constant temperature. Since density increases with pressure, solubility increases with temperature. However, close to the critical point (520 in FIG. 2), the density can drop sharply with a slight increase in temperature. Therefore, close to the critical temperature, solubility often drops with increasing temperature, then rises again. Supercritical-state fluids are completely miscible with each other; thus, a single phase can be guaranteed for a mixture when the critical point of the mixture is exceeded. The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperature and pressures of the two components. For greater accuracy, the critical point can be calculated using equations of state, such as the Peng-Robinson equation or group contribution methods. Other properties such as density can also be calculated using equations of state.